Method for press molding plastics articles, in particular thermoplastic, thermosetting and polyurethane polymeric articles

ABSTRACT

A press for molding thermoplastic, thermosetting, polyurethane polymeric articles includes three workstations arranged side by side and including a central molding station and two lateral stations for cooling and extracting molded articles. Two molds are rigidly connected to one another and controlled by a transfer pusher which alternately moves the molds from the molding station to either of the cooling and extraction stations. A first presser at the central molding station applies a closure pressure necessary for the material injection, while auxiliary presser arranged in each station applies a reduced secondary pressure to the molds at the end of the injection step, during the translatory motion of the mold and during the cooling step.

This is a Division of application Ser. No. 07/118,903 filed Nov. 9, 1987now U.S. Pat. No. 4,861,258 issued Aug. 29, 1989.

BACKGROUND OF THE INVENTION

The present invention relates to a press and a method for moldingthermoplastics, thermosetting, and polyurethane polymeric articles.

In the present description, the term "molding" refers to both knownmethods of injection and injection-compression of fluid polymericmaterial using thermostat-fitted shell molds.

As is known, the current molding technology employs hydraulic presseswith a fixed base and a movable plate accommodating therebetween themold which receives the closing pressure from the movable plate which issubject to the action of one or more fluidodynamic jacks capable ofexerting considerable stresses, for example comprised between 2500 and3500 tons.

The molding process essentially consists of the following operativesteps:

(a) closure and locking of the mold by clamping the same between thebase and the movable plate

(b) advancement of the carriage bearing the injection machine

(c) injection

(d) return of the carriage

(e) unlocking

(f) waiting

(g) translatory motion and opening of the mold.

During the steps (d) to (g), cooling of the molded article occurs,thereafter the mold is opened and the article is extracted. Cooling isperformed while maintaining on the mold a reduced closing pressurerequired for the correct execution of said operative step.

The cooling time, which substantially lasts from the end of theinjection step to the opening of the mold, is considerable, generallycomprised between two and three times the time required by the injectionstep, and is a dead time which considerably affects plant productivityand therefore the cost of the finished product.

If the articles to be molded are small, one tries to contain thenegative effect of the cooling time by using multiple molds so that thedead cooling time is divided among the several articles obtained with asingle molding.

This manner of molding--which besides entails a considerable increase inthe cost of the mold--is not applicable in case of large articles, suchas bumpers for automotive vehicles, dashboards and the like, since themolds, the presses and the injection machines would assume prohibitivecosts and dimensions which are absolutely incompatible both with themovement requirements and with the economy of company management.

SUMMARY OF THE INVENTION

The aim of the present invention is to eliminate these disadvantages,and in particular to provide an improved press and a method involving aconsiderable reduction of the cooling dead times during molding of largearticles, with a consequent considerable increase in the productivity ofthe work cycles and without negative effects on the cost of the molds.

This aim and other objects, which will become apparent from thefollowing detailed description, are achieved by the improved press andmethod, as defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention will become apparentfrom the following detailed description and with reference to theaccompanying drawings, given only by way of non-limitative example,wherein:

FIG. 1 is a schematic plan view of the improved press according to thepresent invention;

FIG. 2 is a perspective view of said press;

FIG. 3 is a partial, enlarged-scale schematic cross section view takenalong line III--III of FIG. 1; and

FIG. 4 is an enlarged-scale cross section view taken along line IV--IVof FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, 10 generally indicates the press and 11 indicates thecarriage bearing the injection machine 12. The press 10 is formed by asturdy framework wherein three workstations are defined, consisting of acentral molding station 20 and of a pair of lateral stations 30A--30B inwhich cooling and extraction of the finished articles occur alternately,as will be described in detail hereinafter. The carriage 11 is alignedwith the central station 20 and is susceptible to move orthogonally withrespect to the framework 10 from a resting position (shown in FIG. 2) toa working position in which the injector nozzle 13 is inserted in thecorresponding injection channel of the mold.

The framework 10 comprises a base 40 common to the three stations and aset of three movable plates, respectively a central one 21 and lateralones 31A--31B. The stem of a fluidodynamic jack 22 acts on the centralplate 21 of the injection station 20, and is adapted to transmit to saidplate the mold closing pressure required in the injection step; the jack22 having such dimensions as to produce a stress comprised between 2500and 3500 tons. The plate 21 is vertically slideable along guiding andretention columns 23 which are part of the framework 10 and the cylinderof the jack 22 is rigidly connected to a fixed reaction plate 24 whichin turn is rigidly associated with the ends of the columns 23.

Similarly, the movable plates 31A--31B are vertically slideable oncorresponding guiding and retention columns 33A--33B of the framework10, at the ends whereof fixed reaction plates 34A--34B are provided.Between the latter and the corresponding movable plates 31A--31B, togglelinkages 35A--35B (or mechanical or electromechanical actuators) areprovided and are intended to cause opening of the mold. The stem of acorresponding fluidodynamic jack 36A--36B acts on the central connectingrod of each linkage and is adapted to deformate the latter in order tovertically shift the movable plate and is also adapted to absorb andtransmit to the corresponding fixed plate 34A--34B the reaction whichacts onto the movable plate due to the action of auxiliary presser meansgenerally indicated at 50 and 60 and adapted to apply to the mold areduced secondary pressure.

The auxiliary presser means 50 are provided on each movable plate 21,31A--31B substantially at each corner, and the auxiliary presser means60 are provided on each portion of plate forming the base 40 and arealigned with the means 50. Said auxiliary presser means 50, 60 (see FIG.3) are each composed of a fluidodynamic jack with a cylinder 51,respectively 61, rigidly associated with the related movable plate 21,31A or 31B and respectively with the base 40, and of a stem, passingthrough a hole in the plate, respectively of the base, at the endwhereof a rotary friction presser element, advantageously a presserroller 52, respectively 62, is supported freely rotatable.

In the position shown in the drawing, two movable molds 70A--70B areinserted between the base 40 on one side and the movable plate 21 of thecentral station 20 and the movable plate 31A of a lateral station 30 onthe other side. The molds 70A, 70B are rigidly associated with oneanother and are each formed by a lower half-shell 71 and by an upperhalf-shell 72. The lower half-shells 71 are slideably movable withrespect to the base 40, being guided in their translatory motion byinverted L-shaped profiled elements 73 rigid with said base. Similarly,the upper half-shells 72 are slideably movable with respect to themovable plates 21, 31A, 31B being guided in their translatory motion byprofiled elements 74 rigid with each movable plate. At least onemovement actuator, advantageously a fluidodynamic pusher 75 (see FIG.1), is provided on the base 40 to alternately provide the translatorymotion of the molds from the central injection station 20 to one or theother of the lateral stations 30A, 30B.

Each mold may be fitted with an extractor which can be providedcompletely on each mold in a known and conventional manner, or cancooperate with means arranged on the base of the framework at eachlateral station to achieve the extraction of the finished article onlywhen the mold is moved to said station.

In the second case, the extraction element 80, provided on the bottom ofthe mold cavity (see FIG. 4, where 70 indicates both the molds 70A,70B), has a stem 81 which traverses the lower half-shell 71 to makecontact with the base 40. A blind longitudinal groove 82 is provided onthe stem 81 (see also FIG. 1), and its upper part is undercut in profileso as to accommodate a counter-shaped head 83 of a rod 84 for raisingthe extractor. The rod 84 in turn passes through the base 40 and isrigidly associated, at its lower part, with a plate 85 which isvertically slideable along guiding and retention columns 86. The plate85 is connected to the stem 87 of a fluidodynamic jack 88 adapted tomove the plate 85 and thus the rod 84 from a lowered resting position toa raised extraction position. In its resting position, the head 83 ofthe rod is aligned with the undercut portion of the groove 82 so thatwhen the mold is moved to a lateral station, the head 83 of thecorresponding transfer rod 84 engages in the aforesaid groove.

Moreover, the molds 70 may be provided with lever-type locking means ofa per se known kind adapted to replace the action of the auxiliarypresser means 50 and 60 above described, though said auxiliary pressermeans are a preferred embodiment of the invention.

The above described press operates as follows.

At the beginning of the cycle, when both molds are empty, the same arearranged one at the central injection station 20 and the other at one ofthe lateral stations, for example at the lateral station 30A asillustrated in FIG. 2. The jacks 22 and 36A are actuated, and jack 22applies to the mold 70B the closing pressure required for injection,while jack 36A causes closure of the mold 70A. Subsequently thepolymeric material is injected into the mold 70B. Once the injection hasbeen completed, the injection carriage 11 is moved backwards and theauxiliary presser means 50 and 60 are actuated, while the pressure ofthe jack 22 is reduced to a value sufficient to oppose the reactionexerted by the auxiliary presser means on the corresponding movableplates 21, 31A. Now the mold transfer pusher 75 is actuated. The mold70B is thus transferred to the lateral station 30B while the mold 70A istransferred to the central injection station. During said translatorymotion, the presser means 50 and 60 maintain on the mold 70B theauxiliary closing pressure, the value whereof is comprised between 15%and 25% of the closing pressure for injection (said auxiliary closingpressure being greater--for example up to 50% of the injection closurepressure--in the case of use of lever-type mold locking means). A newinjection cycle thus begins in the mold 70A while the mold 70B, stillsubject to the abovesaid auxiliary pressure imparted thereto by thepresser means of the plate 31B, cools down.

Once cooling is over, the plate 31B is actuated, after deactivating therelated auxiliary presser means, to open the mold 70B and the extractorelements are actuated and extract the finished article. The plate 31B issubsequently lowered again and the molds are moved in the oppositedirection so that the mold 70A is transferred into the cooling station30A and the mold 70B is transferred into the central injection station20 for another molding cycle.

It is obvious that the press described and its operating method allow asubstantial reduction of cooling dead times, since most of the coolingperiod is employed to inject into the adjacent mold; the overall gain inproductivity (in terms of the amount of articles produced per unit time)being in the order of 75%.

Naturally, the concept of the invention being invariant, the details ofthe execution and the shapes of the embodiment may be extensively variedwith respect to what is described and illustrated by way ofnon-limitative example, without thereby abandoning the scope of theinvention.

We claim:
 1. A method for molding plastic articles comprising:arranginga first mold at a central molding station and a second mold at one oftwo cooling and extraction stations located adjacent said moldingstation on opposite sides thereof, simultaneously molding an article insaid first mold at the central station while applying a pressure by afirst pressure applying means and cooling and extracting a moldedarticle from said second mold at said one of two cooling and extractionstations, positioning second pressure applying means to define anelongated transfer path for said first and second molds, applying aclosure pressure to the molds by means of said second pressure applyingmeans, moving the molds relative to said second pressure applying meansto maintain said closure pressure to bring the second mold into thecentral molding station and the first mold into another of the twocooling and extraction stations, and alternately repeating the aboveoperations by shifting the respective molds simultaneously back andforth between the molding stations and the respective cooling andextraction stations.